I. Field of the Invention
This invention relates to a circuit, system and method for protecting an electronic device against malfunctions caused by undervoltage conditions on the device's power supply.
II. Description of the Relevant Art
It is important when operating an electronic device that the supply which powers the device be carefully monitored. In particular, the supply voltage must be maintained within a tolerance range necessary to ensure proper operation of the device. If the supply voltage deviates outside the tolerance range, then the device may malfunction or, worse yet, may be destroyed.
Typically, there are many ways in which a power supply may fail. The supply may experience a complete loss of power. In such a situation, an auxiliary battery backup supply may be used during the power interruption period. Another type of failure occurs whenever the power supply does not completely fail, but fluctuates outside a specified tolerance range. The power supply may therefore experience either a relatively short or long period of overvoltage or undervoltage. Moreover, if the damping of the load device is insufficient, the power supply will overshoot to overvoltage or undervoltage during its ramping from the undervoltage or overvoltage condition, respectively.
Voltages sufficiently above the tolerance range can cause destruction of the components of the device resulting in a significant amount of attention given to such a situation. It is usual to find overvoltage detection and clamping or shutdown circuits incorporated in the device's power supply in order to prevent these situations. Although overvoltages should never occur, they often do occur and, if they do occur, are remedied within the power supply itself. Undervoltages, on the other hand, occur as a normal part of operation. Undervoltages occur whenever the device is powered on or off, whenever the power supply fails or is disconnected, and whenever the primary source of power drops below some critical level (e.g., whenever brownouts and subsecond interruptions occur associated with AC line-powered devices). When the supply voltage provided to the components making up the device is below a tolerance level for proper operation, the device will malfunction in one or more of the following ways: (1) all operations cease, but resume after the supply voltage returns to within its tolerance level range; (2) all operations cease, and do not resume even after the supply voltage returns to within its tolerance level range; (3) unplanned, improper operations occur, but proper operations resume when the supply voltage returns to within its tolerance level range; and (4) unplanned, improper operations occur, and remain even after the supply voltage returns to within its tolerance range. Situations two through four are at best an inconvenience, and at worst a safety hazard and/or damaging to the device. The first situation is therefore a more suitable way in which to protect the device against the more commonly occurring undervoltage condition.
As opposed to overvoltage conditions, undervoltages present unique problems in maintaining the integrity of the device during the undervoltage and allowing normal operations after the undervoltage. A conventional method used to boost the power supply during a period of undervoltage using a conventional battery backup system is described in U.S. Pat. No. 4,143,283 to Graf et al. Another conventional system using a battery backup to prevent device failure during periods of overvoltage or undervoltage is described in U.S. Pat. No. 4,096,560 to Footh. Such conventional designs which utilize a battery backup system will eventually fail thereby causing an undervoltage whenever the finite life of the battery backup expires. Thus, long periods of undervoltages cannot be prevented with a battery backup system. Conversely, relatively short periods of undervoltages may often go undetected, and, if the undervoltage occurs, even for a short period of time, the electronic device may malfunction.
Undervoltages, often being more difficult to detect and remedy than overvoltages, must either maintain a separate battery boost supply or must incorporate a circuit which sends a reset signal to inhibit the device during periods of undervoltage. In the latter case, the reset signal will inhibit the device and place the device in a halt mode to avoid damage to the device or improper operation. After the reset signal ends, the device begins again with normal operation.
Although protection circuits which provide a reset signal instead of a supply boost achieve the first situation of the four situations stated above and are therefore preferable in operation, many conventional reset-type protection circuits must also maintain a power supply separate from the supply powering the device. In order for the protection circuit to avoid being susceptible to the power supply of the device, a separate, isolated protection circuit supply is needed. An exemplary protection circuit using a battery backup supply separate from the device's power supply is shown in U.S. Pat. No. 4,096,560.
Another problem associated with many conventional protection circuits is that the duration of the reset signal does not extend for a substantial portion of the time period in which the undervoltage occurs. Specifically, the reset signal is placed in an active logic state for a set period of time and is generally used to quickly inhibit the device operation. However, if the undervoltage time period is longer than the reset signal duration, the device may improperly attempt to begin operation.
A still further problem associated with many conventional protection circuits is their inability to maintain the reset signal after the undervoltage period ends. Oftentimes, the power supply feeds a load device which is underdamped. An underdamped load may cause the supply voltage to temporarily rise to an overvoltage condition once the undervoltage condition ends. The overshoot from undervoltage to overvoltage at the end of the undervoltage period may present problems to the electronic device unless the reset signal remains throughout the overshoot period.